It is often desirable to, among other things, be able to develop 3D models of existing facilities such as oil drilling platforms and petrochemical plants. Having an accurate model of the facility will often reduce the costs involved in maintaining and upgrading the facility. Developing a 3D model generally includes capturing data on the facility in its "as built" state, and then manipulating and converting the data so that it ends up as a model in a 3D CAD design system. Once in a 3D CAD system, traditional modeling, editing, and other data manipulation can be performed.
One mechanism for generating a 3D model involves laser scanning and imaging. The process of laser scanning and imaging can be broken into three phases, (1) data capture, (2) view integration, and (3) modeling. In the data capture phase, one or more scans are made from one or more positions (a set of scans from a single position is referred to as a "view"), the scans, in turn, are used to generate one or more views comprising a set of 3D coordinates. In the view integration phase, multiple views are merged to form a "world of points", or "visual database" comprising thousands of 3D coordinates. In the modeling phase, image assembly software is used to turn the "world of points" into CAD-readable geometry, such as planes, cylinders, and surfaces suitable for importation into CAD systems.
Each point in the world of points comprises a 3D (x, y, and z) coordinate, and a reflective intensity value. The 3D coordinate gives the location of a particular point and the intensity value indicates how much of the laser light was reflected back from that point. A display of the world of points is an image of the scanned object. (This is one reason why the world of points is often referred to as a "visual database". As sometimes used herein, a "visual database" is a set of data points wherein each data point comprises a set of coordinates indicating the location of the point relative to a reference point/origin.) But the image lacks the clarity of a photographic image, primarily because the only information available for each point, besides its location, is a reflective intensity value. Having only a reflective intensity value available becomes problematic during the modeling phase.
During the modeling phase, a computer is used to display the world of points. A human operator then selects individual objects visible in the world of points, and provides the additional information required to model those objects in CAD-readable geometry such as planes and cylinders. Because operators are not used to identifying objects based on their ability to reflect a single wavelength of light, identification of objects and model generation is difficult. A comparable situation exists when one looks at an infra-red photograph. Although object identification in an infra-red photograph is possible, it is not so easy as with a standard photograph utilizing light from the visual portion of the spectrum.
An alternative method of 3D model generation is the use of close range photogrammetry. Photogrammetry methods are similar to laser scanning methods in that they generally comprise a data capture, a view integration, and a modeling phase. In photogrammetry, as in laser scanning, view integration results in the creation of a visual database. The visual database created through photogrammetry, however, consists of a set of points with each point having a 3D coordinate and a color value associated with it. Because of the inclusion of color values, displays based on the data in a visual database obtained via photogrammetry tend to be more intuitive than displays of data in a visual database obtained via laser scanning. As a result, the modeling phase of a photogrammetry based method tends to be easier than the same phase of a laser scanning method.
Photogrammetry provides an additional benefit over laser scanning in that the coordinates obtained via laser scanning are "relative", that is, they provide position data only relative to the other points in the visual database. In contrast, coordinates obtained by photogrammetry are "absolute". They provide position data relative to a known outside reference so that the locations of the points in the "real world" are determinable. Moreover, positioning of points using photogrammetry tends to be more accurate (accuracy approximating plus or minus 3 mm) than when using laser scanning (accuracy approximating plus or minus 5 mm). Laser scanning, however, can generally be accomplished in a fraction of the time it takes to perform photogrammetry. Photogrammetry, because of the need to take and develop a large number of photographs, tends to be very time consuming.
In many instances, the speed of laser scanning helps to overcome many of its deficiencies. However, when laser scanning is used to generate a visual database from which 3D models are to be made, the time savings in generating the visual database is insufficient to overcome the increased difficulties posed by limited data. Thus there is a continuing need to improve laser scanning methods to assist in the generation of 3D models based on data obtained through laser scanning.